As is well known, the overflow fusion downdraw process is one of the leading glass making process for making substrates for use in manufacturing display devices. As is also well known, display devices are used in a variety of applications. For example, thin film transistor liquid crystal displays (TFT-LCDs) are used in, among other things, notebook computers, flat panel desktop monitors, LCD televisions, and a variety of communication devices.
Many display devices, such as TFT-LCD panels and organic light-emitting diode (OLED) panels, are made directly on flat glass sheets (glass substrates). To increase production rates and reduce costs, a typical panel manufacturing process simultaneously produces multiple panels on a single substrate.
To take advantage of economics of scale, display manufacturers require ever larger substrates so that more displays and/or larger displays can be manufactured on an individual substrate. The production of larger substrates has proved challenging to the glass manufacturing industry, especially in view of the fact that the substrate thickness is typically less than a millimeter, e.g., 0.7 millimeters historically and currently as low as 0.3 millimeters in some cases.
Particularly challenging has been the problem of managing the behavior of the glass ribbon in the drawing tower. As can be imagined, as the ribbon becomes wider and thinner, it becomes susceptible to complex motions and shapes as it is cooled while moving downward over distances on the order of two stories or more. Because of the demanding standards for substrates used in display devices, the quality portion of the ribbon must remain pristine, thus limiting the potential areas for contact with the ribbon to the ribbon's outer edges (bead portions).
As a consequence of these considerations, the problem of controlling the shape of the ribbon has become a serious engineering challenge. The problem can be formulated as controlling the shape of a moving material without touching the middle of the material where: (i) the material has mechanical properties not unlike those of tissue paper, (ii) the material is being produced continuously in widths of two or more meters, and (iii) the material is subject to complex temperature and stress distributions which dynamically affect the material's mechanical properties. The present disclosure addresses this problem and provides methods for forming and/or stabilizing an out-of-plane bow in a vertical glass ribbon without contacting the quality portion of the ribbon.